Some internal combustion engines are provided with a supercharging system with turbocharger which can increase the power developed by the engine by exploiting exhaust gas enthalpy for compressing the air aspirated by the engine, thus increasing volumetric intake efficiency. An approach to a supercharging system with turbocharger comprises a turbocharger provided with a single common shaft on which there are a turbine arranged along an exhaust duct to rotate at a high speed under the thrust of the exhaust gases expelled by the engine, and a compressor, which is rotated by the turbine and is arranged along the air intake duct to compress the air aspirated by the engine.
The dimensioning and the control mode of a turbocharger are always a trade-off between the needs of the compressor and the needs of the turbine, and between the need to contain the turbo lag and the need to supply a noticeable power increase; therefore, in most cases, neither pneumatic machine works under optimal conditions. Furthermore, the two pneumatic machines must be designed to work together (i.e. always at the same rpm); therefore, the two pneumatic machines cannot be optimized to maximize the respective efficiencies. In order to improve the operation of the turbocharger, it has been suggested to connect a reversible electrical machine to the turbocharger shaft, which reversible electrical machine may work either as an electrical motor to accelerate the compressor as soon as an increase of delivered power is requested, and thus without waiting for the effect of increasing volume and speed of the exhaust gases, or as an electrical generator for “regenerating” the part of mechanical power generated by the turbine and not used by the compressor.
Patent application US2006218923A1 describes an internal combustion engine 12 supercharged by means of a turbocharger 24 provided with a turbine and a compressor, both keyed onto a common shaft onto which an electrical machine 46 is also keyed to be used either as a motor to increase the rpm of the compressor (and thus to cancel out the so-called turbo lag) or as a generator to exploit exhaust gas enthalpy for generating electricity. Patent application US2006218923A1 describes an internal combustion engine 10 supercharged by means of a turbocharger provided with a turbine 22 and with a compressor 24 both keyed onto a common shaft onto which an electrical machine 32 is also keyed to be used either as a motor to increase the revolution speed of the compressor (and thus to cancel out the so-called turbo lag) or as a generator for exploiting the enthalpy of the exhaust gases for generating electricity, which is used to supply an electrical motor 34 keyed onto the drive shaft of the internal combustion engine 10. However, even in these solutions, compressor and turbine are angularly integral and thus always rotate at the same rpm; furthermore, connecting the electrical machine to a shaft of a turbocharger places some problems because the shaft of a turbocharger may rotate at very high speeds (up to 100,000 rpm), which are much higher (also by one order of size) than the rpm of a common electrical machine.
In order to overcome the aforesaid drawbacks, patent application EP2096277A1 describes a turbocharger comprising a turbine, which rotates an electrical generator, and a compressor, which is mechanically independent from the turbine and rotated by an electrical motor. However, the use of two mechanically independent electrical machines considerably increase the weight and dimensions of the turbocharger because each electrical machine must be able to continuously deliver/draw a power equal to the nominal power of the turbocharger. Furthermore, the operating efficiency of the turbocharger is relatively modest because the mechanical power is transferred from the turbine to the compressor by adding up the power losses of the electrical machine (and of the corresponding electronic control device), the power losses of the electrical motor (and of the corresponding electronic control device) and the power losses of the transmission line which connects the electrical generator to the electrical motor.
Patent application JPH0726972 (corresponding to patent JP3386559B2), in the embodiments shown in FIGS. 13, 15 and 16, describes a turbocharger comprising: a turbine 28, which is provided with a shaft 38 mounted in a rotatory manner and connectable to an exhaust duct so as to rotate under the thrust of the exhaust gases; a compressor 26, which is provided with a shaft 36 mounted in a rotatory manner and connectable to an intake duct to increase the pressure of the air fed by the intake duct; and a single electrical machine 136, which is interposed between turbine 28 and compressor 26. The shaft 38 of the 28 and the shaft 36 of compressor 26 are mechanically independent since they do not have any mutual mechanical connection and are thus free to freely rotate with respect to each other. Furthermore, the electrical machine 136 comprises a first rotor 138, which is mechanically connected to the shaft 38 of turbine 28, and a second rotor 137, which is mechanically connected to the shaft 36 of compressor 26 to work as an electromagnetic joint between the shaft 38 of turbine 28 and the shaft 36 of compressor 26. However, the turbocharger described in patent application JPH0726972 has some drawbacks because it is rather large and, above all, the control of the two rotors is complex; i.e. controlling the two rotors independently by means of a common stator is rather complex.